The present invention is directed to a flat motor wherein the axial thickness is minimized, and more particularly to a flat motor which is suitable for use as a fan motor such as used for the radiator fan of a motor vehicle.
The thickness of the various components of a flat motor, such as an armature or field magnet, are necessarily limited when it is desired to reduce the axial thickness of a flat motor. Therefore the magnetic field between the armature and the field magnet will be weakened accordingly.
If the magnetic field between the armature and the field magnet is weak, the output torque will also be weak and the characteristics of such a flat motor will become undesirable. Therefore it is desirable in a flat motor that the magnetic field intensity between the armature and the field magnet be maintained so that the output torque will be efficiently transmitted.
Two methods are available for increasing the magnetic field intensity between the armature and the field magnet. One involves increasing the magnetic flux generated by the field magnet and the other involves the increasing of the magnetic flux generated by the armature.
Increasing the magnetic flux of the armature is not suitable for a flat motor wherein a permanent magnet is used as the field magnet. The maximum field intensity of the permanent magnet is determined by the ferromagnetic materials of the permanent magnet and a strong permanent magnet is usually too big, too heavy, or too expensive. Thus it is more feasible to increase the flux generated by the armature in a flat motor.
Two ways of increasing the flux generated by the armature are increasing the exciting current through the armature and efficiently utilizing the flux generated by the armature. However a big increase in the exciting current will raise the temperature of the armature and deteriorate the reliability of the flat motor. Thus in order to increase the magnetic field intensity between the armature and the field magnet it is most desirable to efficiently utilize the flux generated by the armature.
Many methods of utilizing the flux generated by the armature have been suggested. For example, Japanese laid open patent Application No. 170686/82 discloses a motor which utilizes the flux by fixing the yoke to the armature directly. Japanese laid open patent Application No. 170686/82 also discloses a motor which utilizes the flux by fixing the armature coil to the ferromagnetic member and by fixing the field magnet to the soft magnetic member. However when the rotational speed of a conventional flat motor increases, the temperature of the motor rises, since the rotor generates an eddy current loss.
Reducing the frictional loss between the rotor and the stator is preferable for a flat motor, so as to efficiently transmit the torque generated between the armature and the field magnet. It is also preferable to prevent water droplets, dust or the like from entering into the flat motor.
Japanese laid open patent Application No. 190264/83 and Japanese laid open patent Application No. 170686/82 disclose a motor which includes two bearings provided at opposite ends of the output shaft in order to reduce the friction loss. They also disclose a motor which includes a seal provided between the rotor and the stator. However in this conventional flat motor the thickness is increased because two coaxial bearings are provided at opposite ends of the rotating shaft. Furthermore in a conventional flat motor the temperature of the armature rises because a seal prevents the heat from being radiated to the atmosphere.